File art photo of a firefighting aircraft dropping fire retardant ahead of the River Fire as it burns through a canyon on Aug. 1, 2018 in Lakeport, California. Not the plane involved in state investigation. (Photo by Justin Sullivan/Getty Images)

File art photo of a firefighting aircraft dropping fire retardant ahead of the River Fire as it burns through a canyon on Aug. 1, 2018 in Lakeport, California. Not the plane involved in state investigation. (Photo by Justin Sullivan/Getty Images)

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SACRAMENTO, Calif. (AP) - A firefighter battling the largest wildfire in California history was killed last month when thousands of gallons of flame-suppressing liquid were dropped from an aircraft flying barely above the treetops because spotters mistakenly sent it on a route too close to the ground, according to state investigation findings released Friday.

The pilot and a supervisor flying ahead in a small guide plane led the giant modified Boeing 747 nearly into the trees on Aug. 13 because the pilots failed to recognize that there was a hill in the flight path, according to the Green Sheet report by the state's firefighting agency.

Because of the near ground-level release, the retardant struck with such force it uprooted an 87-foot (27-meter) tree that fell on Matthew Burchett, a 42-year-old battalion chief from Utah helping with the Mendocino Complex Fire north of San Francisco.

Another large tree was snapped by the force of nearly 20,000 gallons (75,700 liters) of liquid and three firefighters were injured, one seriously.

Global SuperTanker Services LLC, which operated the 747, said in a statement that it thought the area was clear of firefighters and "acted within procedural and operational parameters."

Two supervisors -- one in the air and one on the ground -- potentially face discipline or loss of their current positions because of multiple compounding mistakes, said Cliff Allen, president of the union representing California's wildland firefighters.

California Department of Forestry and Fire Protection spokesman Scott McLean said there are ongoing internal investigations into what went wrong.

"We definitely don't want this to ever happen again," McLean said.

The 747 was flying only 100 feet (30 meters) above the treetops, at least 100 feet too low, officials said. The goal is to fly high enough for the retardant to disperse and fall like rain, but at such a low level they said the slurry would have hit the trees at nearly the same speed as the aircraft -- more than 160 mph (260 kph).

The guide pilot "made a `show me' run" for the 747 pilot over the intended path for the retardant drop, and marked the path for the jet with a smoke trail, according to the report.

"Obscured by heavy vegetation and unknown to the (747) pilot, a rise in elevation occurred along the flight path." The ground sloped up about 170 feet (50 meters) in the middle of what otherwise was a flat area, according to the report.

The guide planes have two people aboard, a pilot and an "air tactical supervisor," a specially trained firefighter who directs the pilots of both the guide plane and the airtanker trailing behind.

"He laid down the line and he was directing the tanker and the tanker was following direction," said Allen, the union president.

McLean said spotters have a difficult job because "the ground is very deceptive and very hard to read."

The retardant drops were intended to help secure a firebreak cut through the trees by a bulldozer to stop advancing flames. Burchett and the other three firefighters were working on the hill next to the firebreak when the drop was announced over a radio and firefighters were told to "Clear the area out."

The four did not respond to the warning, though the report says that "when personnel are working under a tree canopy, supervisors must ensure the drop path is cleared."

Allen said the supervisor could face discipline for not getting an acknowledgement that the firefighters were evacuating.

It is not uncommon to have firefighters under retardant drops, McLean said, though he could not say if the four firefighters knew they were in the flight path or why they didn't acknowledge or act on the radioed warning.

A firefighter who can't move out of the way is trained to lie spread-eagled, face down, toward the oncoming aircraft, one hand holding the top of the helmet as it takes the brunt of the impact from the falling slurry and air turbulence that can threaten to lift a firefighter off the ground.

Burchett, a suburban Salt Lake City firefighter, was crushed by the uprooted tree, while the others were stuck by falling tree debris. Two had deep muscle contusions and ligament damage. One also suffered broken ribs, while the fourth firefighter had scratches and abrasions.